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Old 27th December 2006, 11:55 AM   #1
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Default Controller for 401

Hi,
another long-term project is a controller for my Garrard 401.

It will consist of a sine-wave generator at 50Hz and a power amp.
Since now, I only saw circuits that use an output transformator to achieve the desired 220 V.
I don't feel too comfortable with this, as I think that the lower the THD the better, and that is not where OPTs excell at low frequencies.
Do you know of any amps around 15Watts and around 200V output? I don't care wheter if it's a chip amp, mosfet or tubes.
Rüdiger
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Old 27th December 2006, 12:07 PM   #2
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I have measured -80dB distortion on my regenerated power supplies for turntables using a toroidal step-up transformer. The ugly magnetics of the Garrard motor will cause much higher levels than this.

The trick is to use a somewhat over-rated transformer so that the flux in the core is kept within linear bounds.
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Old 27th December 2006, 12:12 PM   #3
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Hi Rüdiger

I may be missing the point here, but why are you worrying about THD in a traffo? Firstly, the average mains traffo is designed to be used at this frequency, so it might be worth firing up RMAA and seeing if there actually is a problem that needs to be fixed. The other issue is the actual effect it will have. Whilst THD will affect the movement of a cone measuring a couple of grammes, with the Thorens you are driving a mechanical system massing kilos, I would expect that any THD would be damped out of existence.

edit: Wrote this reply whilst Mark posted, so I accept that distortion may be measurable, but still am not convinced that it will affect the overall result that much.
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Old 27th December 2006, 12:30 PM   #4
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Hi Al, hi Mark,
I admit that using a bigger transformer will most probably solve most if not all my issues.
OTOH, not to use an additional trafo may reduce cost and effort.

And yes, several people reported, that the key features of a motor controller for a TT are lowest distortion and most stable frequency. So to judge the outcome of an additional controller it shall have those features.
Rüdiger
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Old 27th December 2006, 01:38 PM   #5
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Mark, did you break that distortion down? Just wondering how much is due to just the traffo, and how much for the rest of the circuit.
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